WASHINGTON -- In the course of 15 innings spread over two games during Friday's doubleheader against the Nationals, Mets starting pitchers combined to allow one run, which was unearned. In that span, the Mets outscored the Nationals by 10 runs.

Yet at the end of a marathon day, the Mets managed only a split against the underachieving Nationals. After blowing out the Nationals, 11-0, in the opener, the Mets absorbed a 2-1 walk-off loss in the nightcap.

Ryan Zimmerman launched a solo homer off reliever LaTroy Hawkins with one out in the bottom of the ninth to win it. The loss denied the Mets (46-54) a chance to pull into a virtual tie with Washington for third place in the NL East.

"Fastball, behind in the count 3-and-1, it happens," Hawkins said of Zimmerman's homer.

The Nationals (50-54), who began the season with World Series ambitions, needed to salvage a victory to avoid what would have been an embarrassing sweep.

In his season debut after being sidelined with elbow trouble, 23-year-old Mets righthander Jenrry Mejia gave up seven hits with seven strikeouts in seven innings in the afternoon victory, reinforcing why he once was seen as the organization's top pitching prospect.

In the nightcap, Matt Harvey surrendered an unearned run, five hits and a walk with seven strikeouts in eight innings. He lowered his ERA to 2.11 but came away without a decision.

But it wasn't enough to complete the sweep on a day that began with a drubbing.

In the first game, Daniel Murphy was 4-for-5 with a pair of homers and tied his career high by knocking in five runs. Juan Lagares was 3-for-4 and scored three runs in place of injured leadoff man Eric Young Jr. And with the game out of reach, Ike Davis bashed a three-run shot, his first home run since June 2.

Mejia took care of the rest. In his first major-league action since last September, Mejia overwhelmed the Nationals with a fastball that Hawkins said moved "like a DUI drunk."

But the Mets' offense missed their chances against the Nationals in the nightcap.

With the score tied in the ninth against closer Rafael Soriano, Andrew Brown got a hustle double on a shot to left and moved to third base on a wild pitch. After pinch hitter Davis walked, Lagares fouled out and Murphy flied out.

The Mets took a 1-0 lead in the fourth when John Buck drilled an RBI double over the head of leftfielder Steve Lombardozzi. But that was all the Mets managed against righthander Ross Ohlendorf.

For Harvey, the only blemish came in the fifth, when the Nationals took advantage of Murphy's throwing error to tie the score. With one out and runners on first and second, Wilson Ramos hit a grounder up the middle. Shortstop Justin Turner used his glove to flip to second for the second out, but Murphy threw so wide of first base that his throw struck Ramos. The miscue allowed Jayson Werth to score from second.

Later, Zimmerman won it with one swing.

"That was a tough one to swallow," Murphy said. "I should have made the play."

Notes & quotes: Turner left the second game in the seventh after complaining of shoulder tightness. He earlier collided with Brown while catching a ball in short leftfield . . . The Mets optioned outfielder Kirk Nieuwenhuis to Triple-A Las Vegas to make room for Mejia.